The sound of silence: reducing impact noise.

Many of today's apartments have carpet--even in kitchens and bathrooms. More than fashion, wall-to-wall carpeting, and its underlay, often is relied on to reduce the transmission of sounds--especially impact sounds--between floors. Without some kind of cushion on concrete surfaces, the sounds of pounding feet and moving furniture can travel freely between floors, resulting in unacceptably high levels of thumping, thudding and pitter-patter of what acoustical engineers call structure-borne sound.

As carpets give way to easy-to-clean hard-surface flooring in the face of recurring media images of magnified dust mites, apartment owners can find themselves with a problem: How to enjoy the allergy reducing benefits of hardwood, laminate of ceramic tiles while meeting the technical requirements for impact sound resistance? It's not enough to simply rip out carpet and put down the hard-surface flooring of choice. In fact, that is a guaranteed mute to trouble. To understand why, an apartment owner must know something about how impact sound is transmitted and measured.

Impact Sound

The movement of non-impact or airborne sounds, such as speech, music and household noise, that travel through walls and ceilings is measured on a scale called the Sound Transmission Class (STC). Currently, a minimum rating of 50 STC is required by code between adjoining residential units, although this is not always achieved in the real world. Even when minimum requirements are achieved, many people living with 50 STC between themselves and neighbors consider it an inadequate barrier. But because the physics of impact sounds from fooffall and furniture movements are different from the noises of non-impact sounds, another scale has been developed to measure what acoustical engineers refer to as impact sound.

Impact Insulation Class (IIC) measures the transmission of sound waves caused by an object or objects striking a surface--such as a foot hitting the floor--and transmitting the sound through the material. There is a minimum code rating of 50 IIC for residential applications. This IIC rating is now mandatory where it was once optional. Luxury and premium apartment communities require ratings even higher than 50.

Impact Insulation Class becomes the measurement scale of choice when dealing with sounds created as building parts resonate from the impact of movement and the shifting of loads. Think of the floor on an apartment as a big, concrete drum skin, with the folks below living under that drum. Renters are then at the mercy of the drum thumpers above. That is why everyone in stacked housing situations should be protected from structure borne noise.

Noise Protection

Considering installing hardwood floors in an apartment? Installing it directly on an 8-inch concrete floor might be easy, but it would be a mistake. Wood-on-concrete offers little impact sound insulation, and a sure source of suffering for renters living below. In fact, no hard-surface flooring can be installed over concrete floors alone while still meeting isolation requirements.

While some of the impact sound wave is absorbed by the floor assembly, what appears on the other side of the structure is what we call "noise." Traditional impact insulation products, such as cork sheets, paperboard and synthetic sheet materials, have been around for years; however, nothing matches the impact insulating value of a relative newcomer to sound insulation--recycled rubber.

Made from post-consumer waste, more than 35 million pounds of old tires are converted each year into rubber mats of various thicknesses to be used for impact sound insulation. Rubber, with its natural resiliency and ability to absorb vibrations, provides excellent properties for the attenuation of impact sound. A rubber mat has a higher IIC rating per unit of thickness than alternative materials, and its very firm consistency allows easy installation of new flooring directly over the mat's surface. A 3/16-inch rubber mat between hardwood flooring and 8 inches of concrete produces a 59 IIC. Similar ratings can be achieved with ceramic tiles or vinyl floor surfaces over a variety of commonly used floor assemblies.

While today's society is noisy, sound control need not be the problem it once was for renters. New understanding and innovations in sound reduction have produced products that have impact sound under control. And that's good news for urban planners, acoustical engineers, architects and neighbors everywhere.

Paul Downey P. Eng., is an engineer with Dodge-Regupol Inc. He can be reached at [e-mail] or 416/440-1094 Photos courtesy of Dodge-Regupol.